1. Field of the Invention
The present invention relates to a semiconductor device having a function to control a current to be supplied to a load by a transistor, and a display device including a pixel formed of a current driven display element in which luminance is changed in accordance with a signal, and a signal line driver circuit and a scan line driver circuit for driving the pixel. Further, the present invention relates to a driving method of the display device. Furthermore, the present invention relates to an electronic appliance having the display device as a display portion.
2. Description of the Related Art
A liquid crystal display (LCD) which is a display device formed of a display element such as a liquid crystal is widely used. On the other hand, in recent years, a so-called self-light emitting display device having a pixel which is formed of a display element such as a light emitting diode (LED) has been attracting attention. As a display element used for such a self-light emitting display device, an organic light emitting diode (OLED) (also referred to as an organic EL element, electroluminescence (EL) element, or the like) has been attracting attentions, and they have been used for an EL display and the like. A display element such as an OLED is self-light emitting, therefore, it has advantages such as higher visibility of pixels, no backlight, and higher response compared to a liquid crystal display. Note that the luminance of a display element is generally controlled by a current value flowing through it.
As a driving method for expressing a gray scale of such a display device, there are a digital gray scale method and an analog gray scale method. By the digital gray scale method, a display element is turned on/off by being controlled in a digital manner to express a gray scale. In the case of the digital gray-scale method, the uniformity in luminance of each pixel is excellent; however, only two gray scale levels can be expressed if nothing is done since there are only two states, that is, light emission and non-light emission. Therefore, another method is used in combination to realize a multi-level gray scale. There is an area gray-scale method to express a gray scale by selecting the weighted light emission areas of pixels and a time gray-scale method to express a gray scale by selecting the weighted light emission time. Further, in the case of the digital gray-scale method, a time gray-scale method which is appropriate for achieving high definition is often employed. On the other hand, as the analog gray-scale method, there are a method of controlling the light emission intensity of a display element in an analog manner, and a method of controlling the light emission time of a display element in an analog manner. As the analog gray scale method, often employed is a method to control light emission intensity of a display element in an analog manner. As the method to control light emission intensity in an analog manner, often employed is a current input current driving method which is hardly affected by variations in characteristics of a thin film transistor (hereinafter also referred to as a TFT) of each pixel.
A current input current driven pixel including a unipolar transistor, that is, a transistor having one of a p-channel polarity or an n-channel polarity is disclosed in Patent Document 1 and Non-Patent Document 1.    [Patent Document 1] Japanese Patent Laid-Open No. 2004-021219    [Non-Patent Document 1] SID 04 DIGEST p. 1516-p. 1519